Point well made, and accepted. So, no periastron plunges for Mercury. Darn. It seemed like that would have been quite impressive.neufer wrote:By my calculations when the Sun's corona reaches out to MercuryBDanielMayfield wrote:Mercury is toast for sure. But with its elliptical orbit it should put on an interesting show for a relatively short while as it skims the Sun's surface at periastron. Perhaps it will briefly "plunge" beneath the Sun's photosphere several times only to re-emerge on the outward part of its orbit. But it's doomed, as its orbit decays and the Sun swells.
it will start to intercept about 50 x 1016 kg of material per 88 day orbit.
Since the mass of Mercury is only 33,000,000 x 1016 kg
it should begin to rapidly dissipate kinetic energy over a period of ~660,000 orbits or ~160,000 years.
Note, however, that Mercury will transition into a circular orbit long before it reaches the chromosphere or photosphere.
I wonder where the point will be on the Sun's evolutionary tract where Mercury is just skimming the photosphere? At that point the surface of the poor planet would have to have reached the same temp as the photosphere, wouldn't it?neufer wrote:The sub-solar point on Mercury is already 700 KChris Peterson wrote:It may not. The outer part of a red giant is remarkably tenuous and cool. That combination means that planets may orbit inside the expanded Sun for thousands of years without melting. Of course, it may be a lot rougher ride for Mercury than for Earth or Mars.BDanielMayfield wrote: Mercury will come to have a liquid metallic surface, and thus its name will be even more fitting near its end.
... about half the lava temperature of ~1400 K
The Sun only has to become about 16 times as luminous to turn the sub-solar point on Mercury to ~1400 K lava.